Developmental changes in taste responses from glossopharyngeal nerve in sheep and comparisons with chorda tympani responses

To learn whether there are developmental changes in salt and acid taste responses from the posterior tongue, we recorded from the glossopharyngeal nerve, which innervates taste buds in circumvallate papillae, in sheep fetuses, lambs and adults. Multifiber responses to NH4Cl, KCl, NaCl, LiCl, citric and hydrochloric acids were expressed as ratios, relative to responses for two standard chemicals, NH4Cl and KCl. Response ratios for NaCl and LiCl, relative to either standard, increased during development, but the magnitude of the change was small. KCl elicited very large magnitude responses, relative to NH4Cl, in the youngest fetuses, and then decreased by 50% in stimulating effectiveness. Relative responses to both acids also decreased developmentally. The general shapes of KCl response-concentration functions did not change throughout development; however, in the youngest fetuses, the NH4Cl responses-concentration function was not similar to that in older animals. These developmental changes are different than those for responses from anterior tongue taste buds recorded from the chorda tympani nerve. Anterior tongue responses to NaCl and LiCl change most substantially and those to KCl change very little; acid responses do not change. The developmental differences for anterior versus posterior tongue responses suggest that membrane composition and maturational changes of taste buds in the two locations are not the same. Response-concentration functions from both tongue areas support the proposition that specific membrane components interacting with various salts are added during development.